Phase measuring system

ABSTRACT

A phase measuring system includes apparatus for measuring the difference in phase between a periodic input signal and a reference signal while using the same reference signal as a clock whose periods are counted as a measure of the difference in phase between the reference signal and the input signal. This is accomplished by first multiplying the phase of the input signal and then dividing its frequency and phase by a given amount and comparing this divided signal with the reference signal whose frequency and phase is also divided by the same amount. The two divided signals serve to respectively enable and disable a gate which passes the reference signal to a counter whose count manifests the phase difference between the two signals being compared.

PHASE MEASURING SYSTEM Frederick Bethel Griswold, Roosevelt, NJ.

RCA Corporation, New York, NY.

Aug. 7, 1972 Inventor:

Assigneez Filed:

Appl. No.:

u.s. Cl. 324/83 1) Int. Cl. G01r 25/00, H03d 13/00 'Field of Search324/83 R, 83 D References Cited UNlTED STATES PATENTS 4/1968 Burr 324/83D Primary ExaminerAlfred E. Smith AttrneyEdward J. Norton et al.

5/1968 Lundgreen et a1 324/83 D X 51 Oct. 9, 1973 ABSTRACT A phasemeasuring system includes apparatus for measuring the difference inphase between a periodic input signal and a reference signal while usingthe same reference signal as a clock whose periods are counted as ameasure of thedifference in phase between the reference signal and theinput signal. This is' accomplished by first multiplying the phase ofthe input signal and then dividing its frequency and phase by a givenamount and comparing this divided signal with the reference signal whosefrequency and phase Claims, 1 Drawing Figure A F LEQ 4 l8 DIGITAL PHASE26 E Fd 30 DETECTOR +N 3s s 34 f N [i r Fr/NLO- 28' STOP L |*sTART 24F(l )L F L9. PHASE UP 38/ N DOWN 40 DETECTOR MIXER MIXER COUNTER READOUTPHASE MEASURING SYSTEM The invention herein described was made in thecourse of or under a contract or subcontract thereuncler with theDepartment of the Navy.

This invention relates to phase measuring systems. There are manydifferent ways for measuring the phase between two like frequency waves.One widely used system utilizes a digital phase detector which isenabled by a reference signal and is disabled by an input signal, thedetector measuring the phase difference between the two signals. Whenthe detector is enabled, a clock signal is passed to a counter whichcounts the pulses of the clock that are passed in the interval betweenthe enabling and disabling of the detector. This count manifests thephase difference.

In practice, to obtain an accuracy in a digital meter by a factor of N,the clock rate must also be increased by a factor of N over the rate ofthe reference signal. However, logic speed limitations are ultimatelyreached when upper frequency ranges are utilized such as in themegaI-Iertz range. For example, assume it is desired to measure thephase shift of a megaI-Iertz signal to an accuracy of l nanosecond.Further assume that the 10 megaHertz signal is changing phase in anunknown manner at a rate that would make significant errors during Nperiod averaging. Instantaneous phase measurement therefore would beapplicable. To do this conventionally a l gigaI-Iertz clock would berequired. The present state of the art logic is such that utilization ofsuch a'clock is not practical.

A typical application employing phase measuring devices requiring such ahigh degree of accuracy is in range tone systems utilizing earthorbiting satellites which emit a series of range tones varying infrequency from 100 Hertz to 10 megaHertz. One such system is describedin a copending application entitled Rate Aided Ranging and TimeDissemination Receiver invented by Buntschuh et al., Ser. No. 208,762,filed Dec. 16, 1971, and assigned to the assignee of the presentinvention. In particular, the phase meter of the present invention maybe used in conjunction with a phase tracking system disclosed in mycopending application entitled Analog Phase Tracker, Ser. No. 278,289filed Aug. 7, 1972 and assigned to the same assignee of the presentinvention. Thereceiving station is a moving target which must identifyits location precisely to within one per cent of the period of thehighest frequency range tone. Such precision is beyond the capability ofpresent phase measuring devices.

SUMMARY OF THE INVENTION In accordance with the present invention, anapparatus for measuring the difference in phase between an input signaland a reference signal includes means for expanding the phase of theinput signal by a given amount. Means are coupled to the expanding meansfor altering the frequency and phase of the expanded input signal bydividing thev expandedsignal by the given amount for generating a firstdivided signal. Means are provided for altering the phase and frequencyof the reference signal applied as an input thereto by dividing thereference signal by the given amount for generating a second dividedsignal. Phase measuring means are provided which are responsive to thefirst and second divided signals and the reference signal applied asinputs thereto for generating an output signal manifesting thedifference in phase between the first and second divided signals. Thedifference in phase between the first DETAILED DESCRIPTION In thedrawing phase lock loop 10 comprises a phase detector 14, voltagecontrolled oscillator 16, a divide by N divider 18 and an up mixer 20.Input signal F, Lg is applied as one input to phase detector 14 whichgenerates an error signal e, on lead 22 which is proportional to thedifference in phase between the applied input signal and a signal F,,, lderived by loop 10. Sigrial F [Q is derived by loop 10 such that thephase and frequency of the derived signal tracks the phase and frequencyof the input signal applied to phase detector 14. The output of thevoltage control oscillator, hereinafter VCO 16 is a signal F whosefrequency is substantially the same as the frequency of the input signalF, Lg whose phase is N times the phase of the input signal phase.Voltage control oscillator 16 output is applied to divider 18 whichdivides both the frequency and the phase of the VCO output by an amountN. The output of the divider is a signal F /N LQ whose frequency is f /Nand whose phase is 0. Divider 18 output signal is applied as an input toup mixer 20 which mixes the divider 18 output with a second input F,(1l/N which, when added together, generate a beat signal F LQ whosefrequency and phase substantially match that of the input signal F, LQ'.Error signal e, controls the tracking of F 0 with F, [1 and minimizestracking error 0'-0.

It will be appreciated that up mixer 20 serves to multiply its inputsignals with the net effect of adding and subtracting the inputfrequencies and phases with no effect on the phase of the input signals.Thus, in effect, the phase of the signal at the output of up mixer 20 isequal to and varies in phase at the same rate as the phase of the inputsignal F /N L2, since the other input is at reference with zero phase.

A reference clock 24 generates an output signal F, L12 whose frequency f[2 is the same as the freqiericy of the input signal F, Iiand differingin phase from the input signal by the angle 0. The reference signal F,L0 is applied as an input signal to divide by N divider 26. Divider 26divides by the same amount as divider 18. The output of divider 26 onlead 28 is applied an an input to digital phase detector 30 on lead 34while the output of divider 18 is applied as an input to the phasedetector 30 on lead 36. Digital phase detector 30 serves as a gate forpassing clock pulses on lead 32 to output lead 38 whenever digital phasedetector 30 is enabled. The same point on waveform F,/N

and waveform signal F /N @serve to respectively enable and disable phasedetector 30. The time interval between like transistions of thereference signal F,/N L0 and of the derived signal F lN Lg is the phasedifference between the two signals. Since the clock rate of clock 24 onlead 32 is running at N times the rate of the signals being compared onleads 34 and 36, then the resolution will be l/NF and the count for thephase difference will be 'rNF where r is the phase difference timeinterval.

To provide the second input to up mixer 20 to generate the derivedsignal F 0, the divider 28 output is applied as a first input signal andclock 24 output is applied as a second input signal to down mixer 40.The output of down mixer 40 on lead 42 is a signal at a phase anglesince the mixer, as indicated above, does not change the phase of theinput signals and has a frequency of f,(1l/N). The down mixed signal,when added to the derived signal F /N LQby mixer 20 produces at theoutput of mixer 20 the signal F The output of digital phase detector 30is then applied as an input to the counter 44 which counts the pulsespassed thereto by digital phase detector 30.

Factor N may be any amount, but is preferably an integer in the order of100 wherein the highest frequency of the input signal F, LQ is in theorder of megal-lertz. Ordinarily, the output of clock 24, applied to adigital phase detector such as detector 30, would have to be 100 timesthe highest frequency of the input signal and would in effect be 1gigaHertz. The present state of logic is such that signals at suchfrequencies could not be processed by this logic. By providing thedividing and phase multiplication in accordance with the presentinvention, a reference clock can serve both as a time interval measuringclock and a phase comparing clock both of which having a frequencywithin the limitations of state of art components. Detailed circuitryfor each of the devices of the apparatus of the present invention arewell known and no further description thereof will be provided herein.

The following analysis may prove helpful in further understanding theoperation of the phase meter constructed in accordance with the presentinvention. It can be shown mathematically that the derived waveformsrelate as follows:

F LQ A COS (amt-k9) A, cos m (t+'r) F /N l A cos (ai /N +0 A cos co /N(1+Nr) F, Igg A,,, cos (w N0) A a COS (0,1(li'NT) where 1' is the timedelay of the phase the derived signal F LQwith respect to the phase ofthe reference signal F, From the above equations 1 through 3, it can beseen that time delay 1 passes through divider l8 unchanged and the phaseangle 0 passes through up mixer unchanged.

where 1,. is the loss through the mixer.

The sum or difference is selected by the mixer in accordance with signadjacent the F Lg term. By placing a minus sign adjacent the F: [E inequation (4), the difference term of F [X is selected and the sum termis rejected, thus:

If no minus sign is shown then it is understood that the sum term ischosen and the difference term is rejected. For up mixer 20 let F Lg beF /N L o, F g be F, (1-l/N) mend F, [1 be F, Lg. Substitute these termsin equation (4) and select the sum term.

For phase detector 14 let F Lgbe F, Ii, F Lg be F m, and F [1 be e,which can be shown to be (l AyA /2) [cos(F,F )+(00) where l A,A /2) isreferred to as the phase detector constant K,,. When the phase of theinput signal is not changing F F F, allowing 0 to approach 0' in valueand e, to approach zero. When the phase of the input signal is slowlychanging the deviation of F, is AF, which is (AB/At). The phase of Fwill lag behind the phase of F, producing the dynamic tracking errorwhich is a function of loop bandwidth.

An example of the type of mixer used as down mixer 40, up mixer 20 andphase detector 14 is a 'biphase modulator referred to in thecommunication art as a balanced modulator. This device has a fixed phaseshift of which has no effect on its performance. The above describedrelationships assume that this 90 phase shift occurs throughout. A mixeras used as a phase detector would also have a small amount of offset dueto non-perfect balance which is a function of temperature. This error isgenerally insignificant.

It will occur to those skilled in the art that other arrangements ofboth dividers and multipliers utilizing the principles of frequency andphase division and phase multiplication as described herein, may beemployed in accordance with the present invention. It can also be shownthat the meter, constructed in accordance with the present invention,provides instantaneous phase measurement rather than N period averagingwhile prior art devices provide only average phase. The device of thepresent invention gives instantaneous phase at the time of arrival ofthe zero crossing of the signal where phase is to be measured. This canbe done where the rate of shift in phase or rate of change of phaseerror from the input signal, as compared to the reference signal, is tenparts in a million. That is, for a signal derived from a source movingwith respect to the local reference and having a frequency of 10 mega-Hertz in which there is a Hertz doppler shift, a meter constructed inaccordance with the present invention provides instantaneous phasemeasurement. For example, to measure the phase shift of a 10 mega- Hertzsignal to an accuracy of 1 nanosecond, a clock is provided at 10megaHertz, N is made to belOO and readouts of the counter will occurevery ten microseconds at the zero crossings of the input signal. Thisprovides instantaneous phase measurement for a single cycle of the inputsignal. This apparatus is particularly suitable in the measurement ofpropagation delay in a moving target whose rate of change would makesignificant errors during N period averaging.

What is claimed is:

1. In an apparatus for measuring the difference in phase between aninput signal and a periodic reference the combination comprising:

means for expanding the phase of said input signal applied thereto by agiven amount,

' means coupled to said expanding means for altering the frequency andphase of said expanded input signal by dividing the frequency and phaseof said expanded signal by said given amount for generating a firstdivided signal,

means for altering the phase and frequency of said reference signalapplied as an input thereto by dividing the frequency and phase of saidreference signal by said given amount for generating a second dividedsignal, and

phase measuring means responsive to said first and second dividedsignals and said reference signal applied as an input thereto forgenerating an output signal manifesting the difference in phase betweensaid first and second divided signals, the difference in phase betweensaid first and second divided signals manifesting the difference inphase vbetween said input signal and said reference signal.

2. The combination of claim 1 wherein said phase measuring meansincludes:

a counter responsive to said reference signal for counting the periodsof said reference applied thereto, and gating means for passing saidreference signal to said counter, said gating means being enabled bysaid second divided signal at a given point on the waveform thereof anddisabled by said first divided signal at a given point on the waveformthereof.

3. In an apparatus for measuring the difference in phase between aninput signal and a periodic reference signal, the combinationcomprising:

phase measuring means responsive to said reference signal appliedthereto for generating an output signal manifesting the difference inphase between a first signal input thereto and a second signal inputthereto, phase lock loop responsive to said input signal applied theretoand including a voltage controlled oscillator whose output has a givenfrequency harmonically related to said input signal, at least oneintegral divider and at least one multiplier for altering the phase ofthe voltage controlled oscillator output solely by dividing said outputphase by the divisor of said divider while altering the frequency ofsaid voltage controlled oscillator'output by both dividing the frequencyby said divisor and multiplying said frequency by an integer equal tosaid divisor such that the frequency and phase of said' divided andmultiplied signal is substantially the same as said input signal thenbeing applied to said phase lock loop,

a second integral divider responsive to said reference signal applied asan input thereto for altering the phase and frequency of said referencesignal by dividing said reference signal by said integer, and

means for applying said divided voltage controlled oscillator outputsignal as said first signal input to said phase measuring means, andsaid divided reference signal as said second signal input to said phasemeasuring means.

4. The combination of claim 1 wherein said one multiplier includes firstmixing means for mixing the divided voltage controlled oscillator outputwith a third signal input, and signal deriving means for deriving saidthird signal input from said reference signal.

5, The combination of claim 4 wherein said signal deriving meansincludes second mixing means for mixing the reference signal with saiddivided reference signal to produce said third signal input.

6. The combination of claim 3 wherein said phase measuring meansincludes:

a counter responsive to said reference signal for counting the periodsof said reference signal applied thereto, and

gating means for passing said reference signal to said counter, saidgating means being enabled by said second signal input at a given pointon the waveform thereof and disabled by said first signal input at agiven point on the waveform thereof, the number of reference signalperiods'accumulated in said counter manifesting the phase angle betweensaid first and second signal inputs.

7. In an apparatus for measuring the difference in phase between aperiodic inputsignal having a given repetition rate and a periodicreference signal, the combination comprising:

' signal deriving means responsive to said input signal for deriving asan output thereof a signal whose repetition rate is the same as that ofsaid'periodic input signal and whose phase is a multiple factor of saidperiodic input signal,

means responsive to said derived output signal for dividing thefrequency and phase of said derived output signal by an amount equal tosaid multiple factor for generating afirst divided output signal,

means for dividing by an amount equal to said multiple factor the phaseand frequency of said reference signal for generating a second dividedoutput signal such that the repetition rate thereof is the same as saidfirst divided output signal,

phase measuring means for generating an output signal manifesting thedifference in phase between first and second signal inputs thereto, and

input means for applying said first divided signal as said first signalinput to saidd phase measuring means and said second divided signal assaid second signal input to said phase measuring means, the differencein phase between said first and second divided signals manifesting thedifference in phase between said periodic input and reference signals.

8. The combination of claim 7 wherein said signal deriving meansincludes,

a phase detector for generating an error signal manifesting thedifference in phase between a first signal input thereto and a secondsignal input thereto,

a voltage controlled oscillator for generating as an output thereof saidderived signal in response to said error. signal applied as an inputthereto,

first mixing means for mixing said first divided signal with a thirdsignal input thereto for providing an output signal having a phase andrepetition rate substantially the same as said periodic input signal,and

input means for applying said periodic signal as said first signal inputand said mixing means output signal assaid second signal input 9. Thecombination of claim 8 further including means for generating said thirdsignal as an output thereof including second mixing means for mixing afourth signal input with a fifth signal input, and

second input means for applying said second divided output signal assaid fourth signal input and said reference signal as said fifth signalinput. 10. In combination: 1. A phase lock loop including:

a. a voltage controlled oscillator responsive to an error signal appliedthereto for generating an output signal whose repetition rate is thesame as a periodic input signal applied to said loop and whose phase isa multiple of the phase of said periodic input signal,

b. first dividing means for dividing the phase and repetition rate ofsaid voltage controlled oscillator output signal applied as an inputthereto by a factor equal to said multiple,

c. multiplying means for multiplying solely the repetition rate of saiddivided voltage controlled oscillator output signal applied as an inputthereto, and

d. a phase detector for generating said error signal manifesting thedifference in phase between said periodic input signal and saidmultiplied signal,

2. second dividing means responsive to a reference input signal having areference phase and repetition rate the same as said periodic inputsignal repetition rate, and

3. phase measuring means responsive to said first and second dividedsignals applied as inputs thereto for generating an output signalmanifesting the difference in phase between said first and seconddivided signals which manifest the difference in phase between saidperiodic input signal and said reference signal.

1. In an apparatus for measuring the difference in phase between aninput signal and a periodic reference thE combination comprising: meansfor expanding the phase of said input signal applied thereto by a givenamount, means coupled to said expanding means for altering the frequencyand phase of said expanded input signal by dividing the frequency andphase of said expanded signal by said given amount for generating afirst divided signal, means for altering the phase and frequency of saidreference signal applied as an input thereto by dividing the frequencyand phase of said reference signal by said given amount for generating asecond divided signal, and phase measuring means responsive to saidfirst and second divided signals and said reference signal applied as aninput thereto for generating an output signal manifesting the differencein phase between said first and second divided signals, the differencein phase between said first and second divided signals manifesting thedifference in phase between said input signal and said reference signal.2. second dividing means responsive to a reference input signal having areference phase and repetition rate the same as said periodic inputsignal repetition rate, and
 2. The combination of claim 1 wherein saidphase measuring means includes: a counter responsive to said referencesignal for counting the periods of said reference applied thereto, andgating means for passing said reference signal to said counter, saidgating means being enabled by said second divided signal at a givenpoint on the waveform thereof and disabled by said first divided signalat a given point on the waveform thereof.
 3. In an apparatus formeasuring the difference in phase between an input signal and a periodicreference signal, the combination comprising: phase measuring meansresponsive to said reference signal applied thereto for generating anoutput signal manifesting the difference in phase between a first signalinput thereto and a second signal input thereto, a phase lock loopresponsive to said input signal applied thereto and including a voltagecontrolled oscillator whose output has a given frequency harmonicallyrelated to said input signal, at least one integral divider and at leastone multiplier for altering the phase of the voltage controlledoscillator output solely by dividing said output phase by the divisor ofsaid divider while altering the frequency of said voltage controlledoscillator output by both dividing the frequency by said divisor andmultiplying said frequency by an integer equal to said divisor such thatthe frequency and phase of said divided and multiplied signal issubstantially the same as said input signal then being applied to saidphase lock loop, a second integral divider responsive to said referencesignal applied as an input thereto for altering the phase and frequencyof said reference signal by dividing said reference signal by saidinteger, and means for applying said divided voltage controlledoscillator output signal as said first signal input to said phasemeasuring means, and said divided reference signal as said second signalinput to said phase measuring means.
 3. phase measuring means responsiveto said first and second divided signals applied as inputs thereto forgenerating an output signal manifesting the difference in phase betweensaid first and second divided signals which manifest the difference inphase between said periodic input signal and said reference signal. 4.The combination of claim 3 wherein said one multiplier includes firstmixing means for mixing the divided voltage controlled oscillator outputwith a third signal input, and signal deriving means for deriving saidthird signal input from said reference signal.
 5. The combination ofclaim 4 wherein said signal deriving means includes second mixing meansfor mixing the reference signal with said divided reference signal toproduce said third signal input.
 6. The combination of claim 3 whereinsaid phase measuring means includes: a counter responsive to saidreference signal for counting the periods of said reference signalapplied thereto, and gating means for passing said reference signal tosaid counter, said gating means being enabled by said second signalinput at a given point on the waveform thereof and disabled by saidfirst signal input at a given point on the waveform thereof, the numberof reference signal periods accumulated in said counter manifesting thephase angle between said first and second signal inputs.
 7. In anapparatus for measuring the difference in phase between a periodic inputsignal having a given repetition rate and a periodic reference signal,the combination comprising: signal deriving means responsive to saidinput signal for deriving as an output thereof a signal whose repetitionrate is the same as that of said periodic input signal and whose phaseis a multiple factor of said periodic input signal, means responsive tosaid derived output signal for dividing the frequency and phase of saidderived output signal by an amount equal to said multiple factor forgenerating a first divided output signal, means for dividing by anamount equal to said multiple factor the phase and frequency of saidreference signal for generating a second divided output signal such thatthe repetition rate thereof is the same as said first divided outputsignal, phase measuring means for generating an output signalmanifesting the difference in phase between first and second signalinputs thereto, and input means for applying said first divided signalas said first signal input to said phase measuring means and said seconddivided signal as said second signal input to said phase measuringmeans, the difference in phase between said first and second dividedsignals manifesting the difference in phase between said periodic inputand reference signals.
 8. The combination of claim 7 wherein said signalderiving means includes, a phase detector for generating an error signalmanifesting the difference in phase between a first signal input theretoand a second signal input thereto, a voltage controlled oscillator forgenerating as an output thereof said derived signal in response to saiderror signal applied as an input thereto, first mixing means for mixingsaid first divided signal with a third signal input thereto forproviding an output signal having a phase and repetition ratesubstantially the same as said periodic input signal, and input meansfor applying said periodic signal as said first signal input and saidmixing means output signal as said second signal input.
 9. Thecombination of claim 8 further including means for generating said thirdsignal as an output thereof including second mixing means for mixing afourth signal input with a fifth signal input, and second input meansfor applying said second divided output signal as said fourth signalinput and said reference signal as said fifth signal input.
 10. Incombination: